Directors Retirement Plan - Components of Net Periodic Plan Cost for Retirement Plan (Detail) - Director's Retirement Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 18	$ 14	$ 15
Interest cost	17	22	20
Amortization of gain	0	(2)	0
Net periodic postretirement cost	$ 35	$ 34	$ 35